Materials Science Forum Vol. 879

Abstract: High-strength sheets of Al-5.4Mg-0.2Sc-0.1Zr alloy were produced by equal-channel angular pressing (ECAP) to 12 passes via route B_C at 300 °C (573 K) followed by isothermal rolling at 300 °C (573 K) to a total thickness reduction of 80%. The final sheets with ultra-fine grained (UFG) structure were joined by friction stir welding (FSW). The tensile samples including all of the characteristic FSW microstructural zones were machined perpendicular to welding direction. The material demonstrated excellent superplastic properties in the range of temperatures from 350 (623 K) to 450 °C (723 K) at strain rates ranging from 8.3×10^-3 s^-1 to 3.3×10^-1 s^-1. The base material was found to be prone to abnormal grain growth at the testing temperature. This led to localization of the superplastic deformation in the stir zone section of the joints and thus limited total elongation-to-failure. The relationship between superplastic ductility and microstructure and application of this technique for the fabrication of large-scale superplastic sheets are discussed.

Abstract: Processing bulk nanoscrystalline materials for structural applications still poses a significant challenge, particularly in achieving an industrially viable process. In this context, recent work has proved that complex nanoscale steel structures can be formed by solid reaction at low temperatures. These nanocrystalline bainitic steels present the highest strength ever recorded, unprecedented ductility, fatigue on par with commercial bearing steels and exceptional rolling-sliding wear performances. A description of the characteristics and significance of these remarkable structures in the context of the atomic mechanism of transformation is provided.

Abstract: The effects of Temperature-Strain-Time parameters at the Thermomechanical Processing (TMP) of austenitic, duplex and pearlitic structural steels on the mesostructure formation has been studied based on the laboratory, industrial experiments and TEM analysis. The fragmented dislocation substructure observed in steels with a different carbon, nitrogen, titanium, niobium content as well as recrystallization gave evidence that TMP effects the work-hardening and softening behaviour. The problem of mesostructure appearing in various steels and alloys due to various modes of TMP used hot and hot-warm deformation is discussed. The role of plastic strain in the formation of mesostructure and the relation between the changes in the crystal structure due to TMP and the mechanical properties of the steels are considered.

Abstract: Cubic boron nitride (cBN) particle-dispersed-aluminum (Al) matrix composites were fabricated from the powder mixture composed of cBN, pure Al and Al-5mass% Si alloy in liquid and solid co-existent state by spark plasma sintering (SPS) process. Al/cBN composites were well consolidated by heating at a temperature range between 798 K and 876 K for 1.56 ks by SPS. Microstructures of the composites produced were examined by scanning electron microscopy and the reaction between the cBN particle and the Al matrix was not detected. The relative packing density of the Al/cBN composite was higher than 99 % in a volume fraction range of cBN up to 45 %. The thermal conductivity of the composite increased with increasing the cBN content in the composite in a volume fraction range of cBN between 35 and 45 vol. %. The highest thermal conductivity of 305 W/mK was obtained for Al matrix composite containing 45 vol.% cBN particles.

Abstract: On modern vehicles, the demand is made to be in every respect as efficient as possible. A technical method to increase energetic efficiency is to reduce the vehicle mass through the implementation of lightweight construction measures. The energy consumption decreases by that and the vehicle dynamics behavior of conventionally and alternatively respectively electrically powered vehicles increases. In the department Lightweight and Hybrid Design Methods of the Institute of Vehicle Concepts in Stuttgart in collaboration with 3A Composite Core Materials, a method which allows to realize sandwich structures for automotive structural applications analytically and conceptually, is developed. The development method based on material and component testing and material values would be determined at different loads, for example in pressure and in-plane tests. These values are transmitted into the analytical determination of so called failure mode maps to derive appropriate sandwich structures. With novel sandwich structures the objectives of high structural stiffness and strength are tracked, as well as a high level of energy absorption potential. By function integrating the potential of lightweight construction, depending on the energy absorption per structural weight, can be further increased. Accompanying tests on generic structures are made to validate the failure behavior. Also the influence of core material on the deformation behavior is examined. The results from the tests are transferred to a vehicle front structure of a planned lightweight vehicle of class L7E called "Safe Light Regional Vehicle" (SLRV). The behavior of the structure is examined in static and dynamic tests. The energy absorbing capacity can be further increased by geometric optimization and the use of different core materials. The research on sandwich materials is part of the research project Next Generation Car (NGC) of the DLR and represents in terms of the new vehicle concept SLRV in sandwich design a novel vehicle concept of this joint project.

Abstract: The crystallographic aspects of recrystallization twinning have been characterized in {110}<001> and {110}<112>-oriented single crystals of Cu-2%wt.Al and Al-1%wt.Mn alloys. The samples were plane strain compressed to logarithmic strain of 0.52 and then lightly annealed. SEM/EBSD local orientation measurements on partly recrystallized samples demonstrate the appearance of a specific number of new orientation groups of recrystallized grains, which resulted from rotation of the deformed crystal orientations around axes lying near (but rarely at) selected the <111> directions. Then the primary nucleus can transform through the formation of a first generation recrystallization twin. The most frequent situation was that the twinning plane normal was situated near the rotation axis, around which the crystal lattice of the ‘primary nuclei’ rotates. Based on new algorithm to identify coherent and incoherent twin boundaries the influence of free surface on the intensity of annealing twinning was analyzed.

Abstract: Effects of magnetic field intensity on carbon diffusion in pure iron in paramagnetic ferrite region above the Curie temperature were investigated using carburizing technology. It was found that the magnetic field intensity can significantly affect the carbon diffusion behavior in pure iron in paramagnetic ferrite region in the direction parallel to the magnetic field direction, both the carbon diffusion coefficient and the average carbon diffusion distance increase gradually with the increasing of the magnetic field intensity. Magnetic field annealing obviously promotes the carbon diffusion in pure iron in the direction parallel to the magnetic field in paramagnetic ferrite region above the Curie temperature, and promotion effect increases with the enhancement of the magnetic field intensity. The structure defects made by the magnetic field may cause the increasing of carbon diffusion flux.

Abstract: Experiments were carried to investigate the effect of TiC on the solidification process and microstructure of Al-Pb alloys. It is demonstrated that TiC particles are effective inoculants for the nucleation of the Pb-rich droplets during cooling an Al-Pb alloy in the miscibility gap. A model describing the kinetic behavior of TiC particles in the melt and the liquid-liquid decomposition of Al-Pb was developed. The dissolution, coarsening and precipitation processes of TiC particles as well as the microstructure evolution during the liquid-liquid phase transformation of an Al-Pb alloy were calculated. The numerical results indicate that what determines the refinement efficiency of TiC particles on the Pb-rich droplets/particles is the number density of TiC particles in the melt cooled to the binodal line temperature of the Al-Pb alloy. If the number density of TiC particles in the melt before the beginning of the liquid-liquid decomposition is high enough, the addition of TiC causes a refinement of the Pb-rich droplets/particles and promotes the formation of Al-Pb alloys with a well dispersed microstructure.

Abstract: Custom orthopedic and dental implants may be fabricated by additive manufacturing (AM), for example using electron beam melting technology. This study is focused on the modification of the surface of Ti6Al4V alloy coin-like scaffolds fabricated via AM technology (EBM®) by radio frequency (RF) magnetron sputter deposition of hydroxyapatite (HA) coating. The scaffolds with HA coating were characterized by Scanning Electron microscopy, X-ray diffraction. HA coating showed a nanocrystalline structure with the crystallites of an average size of 32±9 nm. The ability of the surface to support adhesion and the proliferation of human mesenchymal stem cells was studied using biological short-term tests in vitro. In according to in vitro assessment, thin HA coating stimulated the attachment and proliferation of cells. Human mesenchymal stem cells cultured on the HA-coated scaffold also formed mineralized nodules.

Abstract: We have proposed a new extrusion process functionally combined with torsion. Extrusion-torsion simultaneous processing is a very attractive technique for fabricating a rod-shape material with high strength and excellent workability. To improve the hardness, the aging treatment was performed with AZ91D magnesium alloy screw thread-rolled at room temperature using extrusion-torsion simultaneous processing. The distribution of hardness from the tip to center in as thread-rolled screw was modified to uniform distribution by the isothermal aging treatment at 423 K for 460.8 ks. The peak hardness was not depended on the working temperature and rotation speed during extrusion-torsion simultaneous processing. β-Mg₁₇Al₁₂ precipitates are obviously grown in as peak-aged condition comparing with as thread-rolled condition.